Optical disk recording and/or reproducing device, and focusing servomechanism

ABSTRACT

An optical disc recorder and/or player using as a recording medium an optical disc having a plurality of recording layers formed one over another includes an optical pickup having a photodetector to direct a light beam towards any one of the plurality of recording layers and detect a return light beam from a recording layer, and an identifier to compare an output signal from the photodetector with a predetermined reference value and identify, based on a comparison result, which one of the plurality of recording layers formed in the optical disc has focused thereon the light beam from the optical pickup.

TECHNICAL FIELD

The present invention relates to an optical disc recording and/orplayback apparatus and method, for playback of information signalrecorded in an optical disc having a plurality of recording layers ordata layers, and also to a focus servo controller and focus servopull-in method, for controlling the focusing of a light beam to any oneof a plurality of recording layers.

BACKGROUND ART

Conventionally, as a recording medium for a variety of information suchas audio information, video information, etc., optical discs are widelyused from which information recorded therein is read out optically,namely, using a light beam. There have recently been proposed suchoptical discs having formed therein many recording layers for thepurpose of recording a further increased amount of information.

An optical disc having a plurality of recording layers formed therein isknown from the disclosure in the Japanese Unexamined Patent PublicationNo. 8-235641.

Referring now to FIG. 1, there is illustrated in the form of a sectionalview a conventional optical disc, that is, the optical disc having aplurality of recording layers as in the above Japanese Unexamined PatentPublication No. 8-235641. As shown in FIG. 1, the optical disc,generally indicated with a reference 1, includes a first record carrier4 a having a first recording layer 3 a formed on one side of a firstsubstrate 2 a which is light-transmissive and a second record carrier 4b having a second recording layer 3 b formed on one side of a secondsubstrate 2 b which is also light-transmissive. The first and secondrecord carriers 4 a and 4 b are joined to each other with the sides ofthe first and second recording layers 3 a and 3 b having formed thereonthe first and second recording layers 3 a and 3 b, respectively, beingopposite to each other as well as with a light-transmissive adhesivelayer 5 being disposed between the first and second record carries 4 aand 4 b.

Information such as video information or the like is recorded as pitpatterns 6 a and 6 b each defined by tiny pits and lands on the sides ofthe first and second substrates 2 a and 2 b forming the first and secondrecord carriers 4 a and 4 b, respectively, on which the first and secondrecording layers 3 a and 3 b are formed, respectively. The firstrecording layer 3 a is formed on the pit pattern 6 a formed on the firstrecord carrier 4 a by forming a translucent or semi-reflective layer ofSiN, SiO₂ or the like along the pit pattern 6 a by evaporation,sputtering or the like. The first recording layer 3 a reflects 20 to50%, and transmits 30 to 80%, of a light beam incident upon it. Thesecond recording layer 3 b is formed on the pit pattern 6 b formed onthe second record carrier 4 b by forming an aluminum evaporated layer orthe like along the pit pattern 6 b. The second recording layer 3 breflects 60% or more of an incident light, but it will reflect 20 to 50%of a light beam having passed through the first recording layer 3 a andreflected from the second recording layer 3 b.

From the optical disc 1 having the first recording layer 3 a which is atranslucent or semi-reflective as in the above, information recorded inthe second recording layer 3 b can be read by directing a light beam L₂in the same direction as that in which a light beam L₁ is directedtowards the first recording layer 3 a as shown in FIG. 1. In this case,either information recorded in the first recording layer 3 a orinformation recorded in the second recording layer 3 b is selected forreading by changing the focused position of the light beams L₁ and L₂directed towards the optical disc 1.

Also, as the optical disc having a plurality of recording layers, therehas been proposed a one shown in FIG. 2. This example of optical disc isgenerally indicated with a reference 11. As shown in FIG. 2, the opticaldisc 11 includes a first record carrier 14 a having a first recordinglayer 13 a formed on one side of a first substrate 12 a which islight-transmissive and a second record carrier 14 b having a secondrecording layer 13 b formed on one side of a second substrate 12 b whichis also light-transmissive. The first and second record carriers 14 aand 14 b are joined to each other with an adhesive layer 15 of alight-transmissive, ultraviolet-settable resin or the like. They arejoined in parallel with each other so that the second substrate 12 b isdisposed above the first recording layer 13 a. On the second recordinglayer 13 b, there is provided a protective layer 17 for the secondrecording layer 13 b.

Information such as audio information or the like is recorded as pitpatterns 16 a and 16 b each defined by tiny pits and lands on the sidesof the first and second substrates 12 a and 12 b forming the first andsecond record carriers 14 a and 14 b, respectively, on which the firstand second recording layers 13 a and 13 b are formed, respectively. Thefirst recording layer 13 a transmits only a light beam having apredetermined wavelength while reflecting a light beam having any otherwavelength. It is formed on and along the pit pattern 16 a formed on thefirst record carrier 14 a. The first recording layer 13 a is formed tohave a multilayered structure consisting of five layers of Si₃N₄, SiO₂,Si₃N₄, SiO₂ and Si₃N₄, respectively, for example. The first recordinglayer 13 a reflects approximately 34% of a light beam of 635 nm inwavelength while reflecting little of a light beam of 780 nm inwavelength.

On the pit pattern 16 b formed on the second record carrier 14 b, thereis formed the second recording layer 13 b by forming a highly reflectivealuminum evaporated layer or the like along the pit pattern 16 b. Thesecond recording layer 13 b reflects 80% or more of a light beam havingpassed through the first recording layer 13 a. It reflects 84% or moreof a light beam of 780 nm in wavelength, most of which is transmittedthrough the first recording layer 13 a, while reflecting about 38% of alight beam of 635 nm in wavelength, of which about 34% is reflected bythe first recording layer 13 a.

In case of the optical disc 11 constructed as in the above, a light beamis directed from the side of the first substrate 12 a of the firstrecord carrier 14 a to read information recorded in the first and secondrecording layers 13 a and 13 b. At this time, a light beam L₃ of 780 nmin wavelength, used to read the information, will be transmitted throughthe first recording layer 13 a and incident upon the second recordinglayer 13 b, and a portion of the light beam will be reflected as areturn light from the second recording layer 13 b. By detecting thereturn light, information such as the audio information or the likerecorded in the second recording layer 13 b is read. Since the lightbeam L₃ of 780 nm in wavelength is used to play back a so-called compactdisc which is a read-only optical disc of 12 cm in diameter, informationrecorded in the second recording layer 13 b can be read by ageneral-purpose or versatile optical disc player.

Also, a light beam L₄ of 635 nm in wavelength, directed from the side ofthe first substrate 12 a of the first record carrier 14 a, will bereflected from both the first and second recording layers 13 a and 13 bto provide a return light each. More specifically, since the firstreflecting layer 13 a reflects about 34% of a light beam of 635 nm inwavelength, the light beam will pass through the first recording layer13 a and incident upon the second recording layer 13 b to provide areturn light from the second recording layer 13 b. The second recordinglayer 13 b reflects approximately 34% of the light beam incident uponit.

By focusing the light beam L₄ of 635 nm in wavelength on either thefirst or second recording layer 13 a or 13 b of the optical disc 11,information recorded in either the first or second recording layer 13 aor 13 b can be read.

In case of the optical disc 1 in which the first and second recordcarriers 4 a and 4 b are joined to each other with the sides thereof onwhich the first and second recording layers 3 a and 3 b are formed,respectively, being opposite to each other as shown in FIG. 1,information recorded in either the first or second recording layer 3 aor 3 b can selectively be read by focusing a light beam on either thefirst or second recording layer 3 a or 3 b of the optical disc 1. Sincein the optical disc 1, the first and second recording layers 3 a and 3 bare disposed close to each other, an error will possibly take place indetecting the focused position of a light beam to detect the first orsecond recording layer 3 a or 3 b. It will be difficult to positivelyfocus the light beam on the first or second recording layer 3 a or 3 band thus it is not possible to accurately read any desired informationfrom the optical disc 1.

Also it has been proposed to select either the first or second recordinglayer 3 a or 3 b by detecting a difference in reflectance between thefirst and second recording layers 3 a and 3 b to detect on which of thefirst and second recording layers 3 a and 3 b a light beam incident uponthe optical disc 1 is focused. Also in this case, since the first andsecond recording layers 3 a and 3 b has the nearly same reflectance forthe light beam incident upon the optical disc 1, the difference inreflectance for the light beam cannot be used to select either the firstor second recording layer 3 a or 3 b.

In case of the optical disc 11 having the first recording layer 13 awhich transmits only a light beam having a predetermined wavelengthwhile reflecting a light beam having any other wavelength, as shown inFIG. 2, information recorded in the first recording layer 13 a can beread by the versatile optical disc player in which a compact disc is tobe used. To read information recorded in the first and second recordinglayers 13 a and 13 b, a dedicated optical disc player using a light beamof 635 nm in wavelength is necessary. Since the first and secondrecording layers 13 a and 13 b have the nearly same reflectance for thelight beam incident upon the optical disc 11, either the first or secondrecording layer 13 a or 13 b cannot be selected by detecting thedifference in reflectance of the recording layers for the light beam.

As in the above, although the optical discs having so far been proposedhave formed therein a plurality of recording layers for recording anincreased amount of information, desired information cannot be readeasily and accurately because it is difficult to focus a light beamaccurately on a selected one of a plurality of recording layers.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to overcome theabove-mentioned drawbacks of the prior art by providing an opticalrecording medium recorder and/or player for playback of informationrecorded in an optical recording medium having formed therein aplurality of recording layers intended for an increased recordingcapacity, adapted to easily select each of the recording layers andpositively read information recorded in a desired recording layer.

It is another object of the present invention to provide a focus servopull-in method and unit enabling to focus a light beam on a desired oneof a plurality of recording layers formed in an optical recording mediumand positively read information recorded in the recording layer on whichthe light beam is focused.

It is still another object of the present invention to provide anoptical recording medium playback method for selectively reading, with alight beam, a plurality of recording layers formed in the opticalrecording medium.

The above object can be attained by providing an optical disc recorderand/or player including according to the present invention:

an optical pickup having a photodetector to direct a light beam towardsany one of a plurality of recording layers formed one over the other inthe optical disc and detect a return portion of the light beam reflectedback from the recording layer; and

an identifier to compare output signal from the photodetector with apredetermined reference value and identify, based on the comparisonresult, which of the plurality of recording layers in the optical dischas focused thereon the light beam from the optical pickup.

Also the above object can be attained by providing an optical discrecorder and/or player including according to the present invention:

an optical pickup including:

a light source to emit a light beam for incidence upon any one of aplurality of recording layers formed one over the other in the opticaldisc and different in reflectance from each other;

an objective lens to converge the light beam emitted from the lightsource to any one of the plurality of recording layers; and

a photodetector including a plurality of detector blocks to detect areturn portion of the light beam reflected back from the recordinglayer;

a signal generator to generate, from output signals from the detectorblocks of the photodetector, a focus error signal and a signalindicative of a sum of the output signals from the detector blocks; and

a controller including a servo control block to move the objective lensoptical-axially thereof based on the focus error signal from the signalgenerator; and an identifier to identify, based the focus error signaland sum signal from the signal generator, which of the plurality ofrecording layers in the optical disc has focused thereon the light beamfrom the optical pickup.

In the above optical disc recorder and/or player, the identifieridentifies which of the plurality of recording layers in the opticaldisc has focused thereon the light beam from the optical pickup,depending on whether when the focus error signal from the signalgenerator is at a predetermined signal level, the level of the sumsignal from the signal generator is below a threshold for any of theplurality of recording layers formed in the optical disc. Also, theidentifier identifies which of the plurality of recording layers in theoptical disc has focused thereon the light beam from the optical pickup,depending on whether when the focus error signal from the signalgenerator crosses the signal zero level, the level of the sum signalfrom the signal generator is below the threshold for any of theplurality of recording layers formed in the optical disc.

Also the above object can be attained by providing a focus servocontroller including according to the present invention:

a servo control block for servo control to move an objective lensoptical-axially thereof based on a focus error signal generated from anoutput signal from each of detector blocks of a photo detector whichdetects a return portion of a light beam reflected from any of aplurality of recording layers formed one over the other in an opticaldisc and different in reflectance from each other; and

an identifier to identify, based on the focus error signal and the sumof output signals from the detector blocks, which of the plurality ofrecording layers in the optical disc has focused thereon the light beamfrom an optical pickup.

Also the above object can be attained by providing a focus servo pull-inmethod including, according to the present invention, steps of:

generating a focus error signal and a sun signal from output signalsfrom detector blocks of a photodetector to detect a reflected returnportion of a light beam incident through an objective lens upon any of aplurality of recording layers formed one over the other in an opticaldisc and different in reflectance from each other;

identifying, based on the focus error signal and sum signal, which ofthe plurality of recording layers in the optical disc has focusedthereon the light beam from an optical pickup; and

closing a servo loop to move the objective lens optical-axially thereofbased on the focus error signal.

These objects and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription of the preferred embodiments of the present invention whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a conventional optical disc.

FIG. 2 is a sectional view of another conventional optical disc.

FIG. 3 is a sectional view of an optical disc for use with the presentinvention.

FIG. 4 is a block diagram of an optical disc player according to thepresent invention.

FIG. 5 is a circuit diagram of a photodetector to detect a focus errorsignal and a focus error signal generation circuit.

FIG. 6 is a flow chart of operations effected in the focus servo pull-inoperation to focus a light beam to any one of a plurality of recordinglayers provided in the optical disc.

FIG. 7 shows waveforms of RF signal and focus error signal detected fromeach of the recording layers in the optical disc.

BEST MODE FOR CARRYING OUT THE INVENTION

The optical recording medium recording and/or playback apparatus andmethod, optical recording medium focus servo pull-in method and focusservo controller, according to the present invention, will be describedin detail herebelow:

First, an optical recording medium for use with the optical recordingmedium recording and/or playback method, focus servo pull-in method andfocus servo controller according to the present invention, will bedescribed below.

The optical recording medium is an optical disc which is generallyindicated with a reference 21. As shown in FIG. 3, the optical disc 21includes a substrate 22 of a synthetic resin such as light-transmissivepolycarbonate, or glass. The substrate 22 has formed on one side thereofa pit pattern 23 of tiny pits and lands corresponding to recordedinformation. In case the substrate 22 is formed from a synthetic resin,the pit pattern 23 is formed by replication from a stamper duringinjection molding of the substrate 22. In case the substrate 22 isformed from glass, the pit pattern 23 is formed using the 2P (photopolymerization) method. In the 2P method, a light-settable resin such asan ultraviolet-settable resin is charged between a glass substrate anddisc stamper, the light-settable resin is set or cured by ultravioletrays irradiated from the glass substrate side, and thus a pattern ofpits and lands on the stamper is replicated to form the pit pattern 23.

The substrate 22 of the optical disc 21 for use with the opticalrecording medium recording and/or playback method, focus servo pull-inmethod and focus servo controller according to the present invention isformed from a polycarbonate resin by injection molding. As mentionedabove, the substrate 22 has recorded on one side thereof information inthe form of the pit pattern 23 whose a plurality of pits defines aspiral recording track or concentric recording tracks. The substrate 22is formed to have a diameter of 12 cm and a thickness of about 1.2 mmsimilarly to the substrate of a so-called compact disc being aconventional optical disc whose diameter is 12 cm.

As shown in FIG. 3, a first recording layer 24 is formed on the one sideof the substrate 22, on which the pit pattern 23 is formed, to cover thepit pattern 23. The first recording layer 24 is a translucent orsemi-reflective layer which transmits a constant quantity, and reflectsa constant quantity, of the light beam directed from the side of thesubstrate 22. For example, the first recording layer 24 is formed fromsilicone layers of Si₃N₄, SiO₂ and the like, respectively, to athickness of about 100 to 500 nm. Namely, the first recording layer 24is formed from the multiple layers of Si₃N₄, SiO₂ and the like,respectively, to have a multilayered structure on the substrate 22. TheSi₃N₄, SiO₂ and other layers are formed on the substrate 22 by vacuumevaporation or sputtering.

The first recording layer 24 has formed thereon a first intermediatelayer 25 of a light-transmissive ultraviolet-settable resin or the like,and a second recording layer 26 formed on the first intermediate layer25. The first intermediate layer 25 is formed to have a predeterminedthickness which optically separates the first and second recordinglayers 24 and 26 from each other so that they will not be positionedwithin the depth of focus of an objective lens which focuses the lightbeam on the recording layers 24 and 26. More specifically, the firstintermediate layer 25 is formed 30 μm or so thick for example. If thefirst intermediate layer 25 is formed too thin, the first and secondrecording layers 24 and 26 will be positioned within the depth of focusof the objective lens which will further be described later and thus thereflected light from the first recording layer 24 and that from thesecond recording layer 26 cannot be separated sufficiently from eachother. In this case, it will be difficult to accurately detect thereflected light from each of the recording layers 24 and 26. On thecontrary, if the first intermediate layer 25 is formed too thick, aspherical aberration will take place. Taking the above in consideration,the first intermediate layer 25 is formed to have an appropriatethickness.

The first intermediate layer 25 is formed by applying alight-transmissive ultraviolet-settable resin or the like to the firstrecording layer 24 by the spin coat method and irradiating ultravioletrays to the applied resin to cure the latter. Otherwise, the firstintermediate layer 25 may be formed on the first recording layer 24 byapplying the ultraviolet-settable resin or the like to a thickness of 5to 10 μm on the first recording layer 24 and repeating this more thanonce to provide a lamination of the resin layers. Moreover, the firstintermediate layer 25 may be a transparent sheet attached on the firstrecording layer 24.

The first intermediate layer 25 has formed on one side thereof a pitpattern 27 of tiny pits and lands corresponding to information recordedon the second recording layer 26. Similarly to the pit pattern 23, thepit pattern 27 is formed from a plurality of pits defining a spiralrecording track or concentric recording tracks. The pit pattern 27 canbe formed by the 2P method used to form the pit pattern 23 on a glasssubstrate as mentioned above. More specifically, the stamper is pressedto an ultraviolet-settable resin applied by the spin coat method or alaminated transparent sheet, and ultraviolet rays are irradiated fromthe side of the substrate 22 for example to replicate a pit pattern ofthe stamper to the resin or transparent sheet, thereby forming the pitpattern 27 in the first intermediate layer 25.

The second recording layer 26 is formed on and along the pit pattern 27formed on one side of the first intermediate layer 25, thus covering thepit pattern 27. The second recording layer 26 transmits a constantquantity, and reflects a constant quantity, of the light beam directedfrom the side of substrate 22 and having passed through the firstrecording layer 24. Namely, the second recording layer 26 is atranslucent or semi-reflective layer. Similar to the first recordinglayer 24, the second recording layer 26 is formed from silicone layersof Si₃N₄, SiO₂ and the like, respectively, to a thickness of about 100to 500 nm on the first intermediate layer 25. Namely, the secondrecording layer 26 is formed from the multiple layers of Si₃N₄, SiO₂ andthe like, respectively, to have a multilayered structure on the firstintermediate layer 25. The Si₃N₄, SiO₂ and other layers are formed onthe first intermediate layer 25 by vacuum evaporation or sputtering.

Further, a third recording layer 29 is formed over the second recordinglayer 26 with a second intermediate layer 28 formed between the secondand third recording layers 26 and 29. The second intermediate layer 28is formed from a light-transmissive ultraviolet-settable resin or thelike. Similar to the first intermediate layer 25, the secondintermediate layer 28 is formed to have a predetermined thickness whichoptically separates the second and third recording layers 26 and 29 fromeach other so that they will not be positioned within the depth of focusof an objective lens which focuses the light beam on the recordinglayers 26 and 29. More specifically, the second intermediate layer 28 isformed 30 μm or so thick for example.

Similarly to the first intermediate layer 25, the second intermediatelayer 28 is formed by applying a light-transmissive ultraviolet-settableresin or the like to the second recording layer 26 by the spin coatmethod and irradiating ultraviolet rays to the applied resin to cure thelatter. Otherwise, the second intermediate layer 28 may be formed on thesecond recording layer 26 by applying the ultraviolet-settable resin orthe like to a thickness of 5 to 10 μm on the second recording layer 26and repeating this more than once to provide a lamination of the resinlayers. Moreover, the second intermediate layer 28 may be a transparentsheet attached on the second recording layer 26.

The second intermediate layer 28 has formed on one side thereof a pitpattern 30 of tiny pits and lands corresponding to information recordedon the third recording layer 29. Similarly to the pit patterns 23 and27, the pit pattern 30 is formed from a plurality of pits defining aspiral recording track or concentric recording tracks. The pit pattern30 can also be formed by the method used to form the pit pattern 27 onthe first intermediate layer 25.

The third recording layer 29 is formed on and along the pit pattern 30formed on one side of the second intermediate layer 28, thus coveringthe pit pattern 30. Specifically, the third recording layer 29 is formedfrom a highly reflective metal such as aluminum (Al), gold (Au), silver(Ag) or the like to reflect to the optical pickup which will further bedescribed later the light beam having passed through the first andsecond recording layers 24 and 26 formed as translucent orsemi-reflective layers. The third recording layer 29 has formed thereona protective layer 31 formed from an ultraviolet-settable resin or thelike to protect the surface of the third recording layer 29. Theprotective layer 31 is formed by applying the ultraviolet-settable resinor the like to the second intermediate layer 28 by the spin coat methodand then irradiating ultraviolet rays to the applied resin to cure thelatter.

The optical disc 21 for use with the present invention has beendescribed in the foregoing concerning a one whose first and secondintermediate layers 25 and 28 are formed by the 2P method. However, thefirst and second intermediate layers 25 and 28 may be formed by anyother method. For example, sheets having the pit pattern 27 and 30,respectively, formed on one side thereof and also a translucent orsemi-reflective layer or a highly reflective metal layer of aluminum(Al), gold (Au), silver (Ag) or the like formed along the pit patterns27 and 30, respectively, may be formed as the second or third recordinglayer 26 or 29, and they may be joined one over the other to the upperside of the first recording layer 24.

From the optical disc 21 for use with the present invention, informationrecorded in any of the first to third recording layers 24, 26 and 29 isread by directing a light beam from the side of the substrate 22 anddetecting a return light reflected by that recording layer 24, 26 or 29by a photodetector of an optical pickup disposed at the side of thesubstrate 22. To this end, the first recording layer 24 is formed from atranslucent or semi-reflective layer which transmits a predeterminedquantity of the directed light beam towards the second and thirdrecording layers 26 and 29, and the third recording layer 29 is from ahighly reflective layer which reflects the light beam having passedthrough the first and second recording layers 24 and 26 and incidentthereupon.

In the optical disc 21, the first recording layer 24 is formed toreflect about 5% of the light beam incident upon the substrate 22 as areturn light L₅. The second recording layer 26 is formed to reflectabout 20% of the light beam incident upon the substrate 22 as a returnlight L₆. The third recording layer 29 is formed to reflect about 60% ofthe light beam incident upon the substrate 22 as a return light L₇.

Owing to the differences in reflectance among the first, second andthird recording layers 24, 26 and 29, there can be large differences inquantity of light among the return portions L₅, L₆ and L₇ of the lightbeam, incident upon and reflected by the first, second and thirdrecording layers 24, 26 and 29. Namely, by detecting the differencesamong the return portions L₅, L₆ and L₇ of the incident light beam uponthe optical disc 21, it is possible to easily detect which of the first,second and third recording layers 24, 26 and 29 has focused thereon thelight beam incident upon the optical disc 21, as will further bedescribed later.

For positive detection of which of the first, second and third recordinglayers 24, 26 and 29 has focused thereon the incident light beam on theoptical disc 21 by detecting the differences in quantity of light amongthe return portions L₅, L₆ and L₇ of the incident light beam from thefirst, second and third recording layers 24, 26 and 29, there shoulddesirably be a difference of 15% or more in reflectance among the first,second and third recording layers 24, 26 and 29. Therefore, the first,second and third recording layers 24, 26 and 29 are formed for thedifference in ratio of return portion of the incident light beams amongthem to be 15% or more.

When the third recording layer 29 reflects about 60% of the light beamincident upon the substrate 22 as the return light L₇, it can be read byan optical disc player which requires the recording layer of aconventional optical disc to have a high reflectance, such as aso-called compact disc player. The optical disc 21 having such a highlyreflective recording layer for use with the present invention isinterchangeable with a conventional optical disc having such a highlyreflective recording layer, namely, the so-called compact disc, and canbe played with a widely prevailing optical disc player, namely, theso-called compact disc player.

Note that although the optical disc 21 for use with the presentinvention has constructed to have the three recording layers, it may beconstructed to have more than two or three recording layers. In thiscase as well, the recording layers should desirably be formed thedifference in ratio of the return portion of the incident light beamamong them to be 15% or more.

Next, an optical disc player capable of reading information recorded inthe optical disc 21 having the first, second and third recording layers24, 26 and 29 as having been described the foregoing, will be described.

Referring now to FIG. 4 for example, an optical disc player isillustrated in the form of a block diagram. As shown, the optical discplayer includes an optical pickup 50 to emit a light beam towards theoptical disc 21 and provide a read signal (will be referred to as “RFsignal” hereinafter) generated based on a return light from the opticaldisc, a sync detection circuit 61 to detect a sync component of the RFsignal from the optical pickup 50, a phase lock loop circuit 62 (will bereferred to as “PLL circuit” hereinafter) to extract a clock signalsynchronous with the RF signal from the optical pickup 50, and a digitalsignal processing circuit 63 to process data from the sync detectioncircuit 61 in a predetermined manner and provide a read data.

The optical pickup 50 includes a laser source such as a semiconductorlaser element or the like, objective lens, actuator to move theobjective lens optical-axially and radially of the optical disc 21,photodetector which will further be described) (these elements are notshown), etc. Output signal from the photodetector is supplied to an RFcircuit 60. The RF circuit 60 generates and provides as an output an RFsignal based on the output signal from the optical pickup 50 and a focuserror signal based on the astigmatic method using a cylindrical lensdisposed in the optical path of the return light reflected from theoptical disc 50 for example and a tracking error signal based on theso-called 3P method for example. The PLL circuit 62 reproduces a clocksignal synchronous with a clock component of data included in the RFsignal supplied from the RF circuit 60 and supplies it to the syncdetection circuit 61 and digital signal processing circuit 63. The syncdetection circuit 61 detects a sync signal included in the RF signalsupplied from the RF circuit 60 based on the clock signal from the PLLcircuit 62, pulls in the synchronization, and supplies the RF signalhaving pulled in the synchronization to the digital signal processingcircuit 63. The digital signal processing circuit 63 uses the clocksignals supplied from the PLL circuit 62 and a crystal oscillator 64,respectively, to demodulate the supplied RF signal by the modulatingmethod corresponding to the EFM (eight to fourteen modulation) methodused at the time of data write to the optical disc, while making anerror correction by an error correction method corresponding to theencoding method, such as the interleave, Reed-Solomon code or the like,used at the time of data write to the optical disc, thereby generating aread digital signal and providing it as an output. The read digitalsignal may be supplied to a D/A converter (not shown) as necessary toprovide it as an analog signal.

As shown in FIG. 4, the optical disc player further includes a trackingservo control circuit 65 to provide a tracking servo control based onthe tracking error signal from the RF circuit 60, a rotation servocontrol circuit 67 to rotate the optical disc 21 at a constant linearvelocity for example based on the reference clock signal from thecrystal oscillator 64 and clock signal from the PLL circuit 62, aspindle motor 68 to rotate the optical disc 21, and a focus servocontrol circuit 69 to provide a focus servo control based on the focuserror signal from the RF circuit 60.

The rotation servo control circuit 67 generates and supplies to aspindle motor 68, based on the reference clock signal from the crystaloscillator 64 and clock signal from the PLL circuit 62, such a drivecurrent that the velocity of the laser spot from the optical pickup 50in relation to the optical disc 21 will be a constant linear velocity.The spindle motor 68 drives to rotate the optical disc 21 at theconstant linear velocity.

The optical disc player further includes a sub-code detection circuit 66which detects a sub-code included in the RF signal from the RF circuit60 to detect a current track position of the optical pickup 50 inrelation to the optical disc 21 and supplies information indicative ofthe detected track position to a feed mechanism (not shown) of theoptical pickup 50. The tracking servo control circuit 65 provides aservo control based on the tracking error signal from the optical pickup50 to move the objective lens of the optical pickup 50 in such adisc-radial direction that the tracking error signal will be “zero”,namely, in a direction perpendicular to the optical axis of theobjective lens. Thereby, the laser spot directed from the optical pickup50 to the optical disc 21 will accurately scan a recording track on theoptical disc 21. The tracking servo control circuit 65 will open thetracking servo loop when accessing the recording track, move the opticalpickup 50 radially of the optical disc 21 by the feed mechanism (notshown) based on the information indicative of the track position,supplied from the sub-code detection circuit 66, and closes the trackingservo loop after the optical pickup 50 reaches the track positionindicated by the information supplied from the sub-code detectioncircuit 66.

As a result of the operation of an operation unit (not shown) by theuser, or based on a command from a host computer, a system controllerinstructs the focus servo control circuit 69 to read data from any ofthe three recording layers, first, second and third (24, 26 and 29,respectively, provided in the optical disc 21.

The optical pickup 50 includes a photodetector 51. As shown in FIG. 5for example, the photodetector 51 is comprised of four detector blocks51A, 51B, 51C and 51D. The RF circuit 60 includes an amplifier 52 to addtogether outputs from the detector blocks 51A and 51C, respectively, anamplifier 53 to add together outputs from the detector blocks 51B and51D, respectively, an amplifier 54 to add together outputs from theamplifiers 52 and 53, respectively, and a differential amplifier 55 tosubtract the output of the amplifier 53 from that of the amplifier 52.

The amplifier 52 adds together the output from the detector block 51A(this output will be referred to as “A” hereinafter) and that from thedetector block 51C (this will be referred to as “C” hereinafter), andsupplies the result (A+C) to the amplifiers 54 and 55, respectively. Theamplifier 53 adds together the output from the detector block 51B (thisoutput will be referred to as “B” hereinafter) and that from thedetector block 51D (this output will be referred to as “D” hereinafter),and supplies the result (B+D) to the amplifiers 54 and 55. The amplifier54 adds together the output (A+C) from the amplifier 52 and that (B+D)from the amplifier 53, and supplies the result (A+B+C+D), that is, theRF signal to the sync detection circuit 61 as in the above and also tothe focus servo control circuit 69. On the other hand, the differentialamplifier 55 subtracts the output (B+D) of the amplifier 53 from theoutput (A+C) of the amplifier 52 and supplies the result {(A+C)−(B+D)},that is, the focus error signal, to the focus servo control circuit 69.

After moving optical-axially the objective lens for a focusing near arecording layer in the optical disc 21, which is designated by thesystem controller, the focus servo control circuit 69 provides a focusservo control based on the focus error signal from the amplifier 55 sothat the focus error signal will be zero. For example, the focus servocontrol circuit 69 will generate a focus servo signal based on thesupplied focus error signal and supplies it to an actuator of theoptical pickup 50, and the actuator will move the objective lensoptical-axially of the objective lens.

Next, operation of pulling in the focus servo control of the focus servocontrol circuit 69 to each of the recording layers 24, 26 and 29 in theoptical disc 21 will be described below with reference to a flow chartin FIG. 6 and an RF signal and so-called S curve shown in FIG. 7.

The first, second and third recording layers 24, 26 and 29 in theoptical disc 21 reflects 5%, 20% and 60%, respectively, of the lightbeam incident upon the optical disc 21 through the objective lens of theoptical pickup 50 as having previously been described. So, when theobjective lens is moved in one direction parallel to the optical axis ofthe objective lens near and over the focused position of the light beamon each recording later, the RF signal and S curve of the focus errorsignal for each of the first, second and third recording layers 24, 26and 29 will have three peak levels and amplitudes, small, medium andlarge, respectively, corresponding to their respective reflectance asshown in FIG. 7.

After the focus servo control circuit 69 moves the objective lensoptical-axially thereof so that the light beam is focused near one ofthe recording layers in the optical disc 21, that is designated by thesystem controller (not shown), it will move the objective lens in onedirection parallel to the optical axis thereof over the focused positionof the light beam. At step S1, the focus servo control circuit 69 judgeswhether the RF signal generated at this time is smaller than apredetermined threshold for the designated recording layer, namely, themaximum value for that recording layer. If the focus servo controlcircuit 69 determines the RF signal to be smaller than the threshold(YES), it will go to step S2. On the contrary, when the focus servocontrol circuit 69 determines the RF signal not to be smaller than thethreshold (NO), it will go back to step S1. Note that the method formoving the objective lens to near a desired one of the recording layersis to supply from the focus servo control circuit 69 to the actuator ofthe optical pickup 50 a signal, for example, a DC signal, to move theobjective lens optical-axially thereof. In this embodiment, a DC signallevel is previously recorded for each of the recording layers 24, 26 and29 in the optical disc 21 and the signal level is selected by the systemcontroller (not shown) for one of the recording layers 24, 26 and 29 onwhich the light beam is focused by the objective lens.

At step S2, the focus servo control circuit 69 determines anoptical-axial position of the objective lens corresponding to point atwhich each of the S curve of the focus error signal crosses the level 0(this position will be referred to as “zero-cross position”hereinafter), and then goes to step S3.

At step S3, the focus servo control circuit 69 will judge whether the RFsignal which will be when the objective lens is at the zero-crossposition is smaller than a threshold TH₁ for reading data from the firstrecording layer 24 for example, smaller than a threshold TH₂ for readingdata from the second recording layer 26 for example, and smaller than athreshold TH₃ for reading data from the third recording layer 29 forexample. When any of the judgments is OK, the focus servo controlcircuit 69 takes that data is to be read from the recording layer inconsideration, and closes the focus servo loop. The result of judgmentis supplied to the system controller (not shown) for use in a servo gaincontrol and the like. When a desired one of the recording layers 24, 26and 29 of the optical disc 21 is thus selected, the selected recordinglayer is subjected to focus servo control and tracking servo control,and data recorded in the selected recording layer is read. The focusservo control is effected with the above-mentioned DC signal added as anoffset signal component to a focus error signal from the optical pickup50 and supplied to the actuator of the optical pickup 50.

If the RF signal which will be-when the objective lens is at thezero-cross position is determined, at step S3, not to be smaller thanthe threshold for the selected recording layers 24, 26 or 29, the focusservo control circuit 69 will go back to step S1 and repeat theoperations at steps S1 to S3.

With the rotation, tracking and focus controlled by the respective servocontrol circuits, respectively, as in the above, the focus servo loopfor focusing the light beam on a desired one of the recording layers 24,26 and 29 in the optical disc 21 is closed. After the focus servo isthus pulled in, information such as audio information and the likerecorded in the desired one of the recording layers 24, 26 and 29 in theoptical disc 21 is read. At this time, a selection is made between thegains of the tracking and focus servos by the system controller (notshown) based on the result of judgment on which of the recording layers24, 26 and 29 in the optical disc 21 has focused thereon the incidentlight beam on the optical disc 21.

Note that the present invention is not limited to the embodiment havingbeen described in the foregoing, but upper and lower limits such asthresholds TH₁ and TH₁′, TH₂ and TH₂′, and TH₃ and TH₃′ may be set forthe RF signal from each of the recording layers 24, 25 and 26 as shownin FIG. 7. For example, the object lens may be moved for the light beamto focused between the first recording layer 24 located at the side ofthe optical disc 21 upon which the light beam is incident and the thirdrecording layer 28 located over the first recording layer 24 and thenmoved optical-axially hereof instead of moving the objective lensoptical-axially thereof to near the focused position of the light beamon a first designated recording layer, to judge which of the recordinglayers has focused thereon the incident light beam upon the optical disc21.

Also, the method for generation of the focus error signal is not limitedto the aforementioned astigmatic method but may be any other one.

The present invention has been described in the above concerning theoptical disc 21 having the three recording layers. Also with an opticaldisc having two recording layers, the focus servo control of the lightbeam for each of the recording layers can be pulled in as in the above.The light beam to be focused on a selected recording layer is subjectedto a focus servo control and tracking servo control to read datarecording in that recording layer.

Of course, the present invention may be applied to an optical discplayer as well as to an optical disc recorder adapted to write data toan optical disc having formed therein a plurality of recording layers atleast one of which is formed from a data-recordable material includingdata-rewritable type materials such as a magneto-optical recordingmaterial and phase-change recording material, and a recordable typerecording material using an organic dye.

Industrial Applicability

As having been described in the foregoing, according to the presentinvention, a light beam is emitted from a light source to an opticaldisc having a plurality of recording layers formed one over the other, areturn light from any of the recording layers in the optical disc isdetected by a photodetector, and a peak level of the output from thephotodetector is compared with a threshold set for each of the recordinglayers in the optical disc to determine which of the recording layers inthe optical disc the return light comes from. When the light beam isfocused on that recording layer, a focus servo loop is closed to readinformation recorded in that recording layer. Thus, it is possible topositively read information recorded in a desired recording layer.

Also, the optical recording medium for use with the present inventionhas at least first and second recording layers formed one over the otheron a light-transmissive substrate. The first and second recording layersare formed to be so different in reflectance from each other that it canbe known based on the signal level based on the return portion of theincident light beam upon the optical disc which of the recording layershas the incident light beam focused thereon. Thus, these recordinglayers can easily be discriminated from each other by detecting thereturn light from each of them.

What is claimed is:
 1. An optical disc recorder and/or playercomprising: an optical pickup having a photodetector for directing alight beam towards any one of a plurality of recording layers eachhaving different reflectances and formed one over another in an opticaldisc, and for detecting a peak level of a return portion of the lightbeam reflected back from one of the plurality of recording layers andproducing an output signal; and an identifier for comparing the peaklevel detected by the photodetector to pre-set upper and lowerthresholds set for each of the plurality of recording layers and foridentifying, based on a comparison result, the one of the plurality ofrecording layers in the optical disc having focused thereon the lightbeam from the optical pickup.
 2. The optical disc recorder and/or playeras set forth in claim 1, wherein the identifier identifies whether theoutput signal from the photodetector is below a predetermined thresholdfor any one of the plurality of recording layers formed in the opticaldisc.
 3. The optical disc recorder and/or player as set forth in claim2, further comprising a signal generator to generate a focus errorsignal from the output signal from the photodetector of the opticalpickup; the identifier identifying, based on the focus error signal fromthe signal generator and the output signal from the photodetector, whichone of the plurality of recording layers in the optical disc has focusedthereon the light beam from the optical pickup.
 4. The optical discrecorder and/or player as set forth in claim 3, wherein the identifieridentifies which one of the plurality of recording layers in the opticaldisc has focused thereon the light beam from the optical pickup,depending on whether when the focus error signal from the signalgenerator is at a predetermined signal level, a level of a sum signalfrom the signal generator is below a threshold for any one of theplurality of recording layers formed in the optical disc.
 5. The opticaldisc recorder and/or player as set forth in claim 4, wherein theidentifier identifies which one of the plurality of recording layers inthe optical disc has focused thereon the light beam from the opticalpickup, depending on whether when the focus error signal from the signalgenerator crosses a signal zero level, the level of the sum signal fromthe signal generator is below the threshold for any one of the pluralityof recording layers formed in the optical disc.
 6. The optical discrecorder and/or player as set forth in claim 3, wherein thephotodetector includes the plurality of detector blocks, and the signalgenerator generates a focus error signal based on detection signals fromthe plurality of detector blocks in the photodetector.
 7. An opticaldisc recorder and/or player comprising: an optical pickup including: alight source to emit a light beam for incidence upon any one of aplurality of recording layers formed one over another in an opticaldisc, wherein the plurality of recording layers have differentrespective reflectances; an objective lens to converge the light beamemitted from the light source to any one of the plurality of recordinglayers; and a photodetector including a plurality of detector blocks todetect a return portion of the light beam reflected back from one of theplurality of recording layers; a signal generator to generate, fromoutput signals from the detector blocks of the photodetectorcorresponding to the return portion of the reflected light beam, a focuserror signal and a peak level of a sum of the output signals from thedetector blocks; a controller including a servo control block to movethe objective lens optical-axially thereof based on the focus errorsignal from the signal generator; and an identifier to identify, basedon the focus error signal and the peak level of the sum signal from thesignal generator, the one of the plurality of recording layers in theoptical disc having focused thereon the light beam from the opticalpickup by comparing the peak level generated by the signal generator topre-set upper and lower thresholds set for each of the plurality ofrecording layers.
 8. The optical disc recorder and/or player as setforth in claim 7, wherein the identifier judges whether the sum signalfrom the signal generator is below a predetermined threshold for any oneof the plurality of recording layers in the optical disc.
 9. The opticaldisc recorder and/or player as set forth in claim 8, wherein theidentifier identifies which one of the plurality of recording layers inthe optical disc has focused thereon the light beam from the opticalpickup, depending on whether when the focus error signal from the signalgenerator is at a predetermined signal level, a level of the sum signalfrom the signal generator is below a threshold for any one of theplurality of recording layers formed in the optical disc.
 10. Theoptical disc recorder and/or player as set forth in claim 9, wherein theidentifier identifies which one of the plurality of recording layers inthe optical disc has focused thereon the light beam from the opticalpickup, depending on whether when the focus error signal from the signalgenerator crosses a signal zero level, the level of the sum signal fromthe signal generator is below a threshold for any one of the pluralityof recording layers formed in the optical disc.
 11. The optical discrecorder and/or player as set forth in claim 10, wherein the controllercloses a servo loop of a servo control block based on an identificationresult.
 12. A focus servo controller comprising: a servo control blockfor servo control to move an objective lens optical-axially thereofbased on a focus error signal generated from an output signal from aphotodetector which detects a return portion of a light beam from anoptical pickup reflected from any one of a plurality of recording layersformed one over another in an optical disc, wherein the plurality ofrecording layers have different respective reflectances; and anidentifier to identify, based on the focus error signal and a peak levelof a sum of output signals from detector blocks of the photodetector,which one of the plurality of recording layers in the optical disc hasfocused thereon the light beam from the optical pickup by comparing thepeak level detected by the photodetector to pre-set upper and lowerthresholds set for each of the plurality of recording layers.
 13. Thefocus servo controller as set forth in claim 12, wherein the identifieridentifies whether the output signal from the photodetector is below apredetermined threshold for any one of the plurality of recording layersformed in the optical disc.
 14. The focus servo controller as set forthin claim 13, wherein the identifier identifies which one of theplurality of recording layers in the optical disc has focused thereonthe light beam from the optical pickup, depending on whether when thefocus error signal from the signal generator is at a predeterminedsignal level, a level of the sum signal from the signal generator isbelow a threshold for any one of the plurality of recording layersformed in the optical disc.
 15. The focus servo controller as set forthin claim 14, wherein the identifier identifies which one of theplurality of recording layers in the optical disc has focused thereonthe light beam from the optical pickup, depending on whether when thefocus error signal from the signal generator crosses a signal zerolevel, the level of the sum signal from the signal generator is belowthe threshold for any one of the plurality of recording layers formed inthe optical disc.
 16. The focus servo controller as set forth in claim15, wherein the servo control block closes a servo loop of the servocontrol block based on an identification result.
 17. A focus servopull-in method comprising steps of: generating a focus error signal anda sum signal from output signals from detector blocks of a photodetectorto detect a reflected return portion of a light beam incident through anobjective lens of an optical pickup upon any one of a plurality ofrecording layers formed one over another in an optical disc, wherein theplurality of recording layers have different respective reflectances;identifying, based on the focus error signal and a peak level of the sumsignal, which one of the plurality of recording layers in the opticaldisc has focused thereon the light beam from the optical pickup bycomparing the peak level detected by the photodetector to pre-set upperand lower thresholds set for each of the plurality of recording layers;and closing a servo loop to move the objective lens optical-axiallythereof based on the focus error signal.
 18. The focus servo pull-inmethod as set forth in claim 17, wherein the identification is effectedafter moving the objective lens optical-axially thereof so that thelight beam is focused near any one of the plurality of recording layersformed in the optical disc.
 19. The focus servo pull-in method as setforth in claim 18, wherein it is judged whether the sum signal from thesignal generator is below a predetermined threshold for any one of theplurality of recording layers in the optical disc.
 20. The focus servopull-in method as set forth in claim 19, wherein it is identified whichone of the plurality of recording layers in the optical disc has focusedthereon the light beam from the optical pickup, depending on whetherwhen the focus error signal from the signal generator is at apredetermined signal level, a level of the sum signal from the signalgenerator is below a threshold for any one of the plurality of recordinglayers formed in the optical disc.
 21. The focus servo pull-in method asset forth in claim 20, wherein it is identified which one of theplurality of recording layers in the optical disc has focused thereonthe light beam from the optical pickup, depending on whether when thefocus error signal from the signal generator crosses a signal zerolevel, the level of the sum signal from the signal generator is below athreshold for any one of the plurality of recording layers formed in theoptical disc.
 22. An optical disc recording and/or playback methodcomprising steps of: generating a sum signal from output signals from aplurality of detector blocks in a photodetector to detect a reflectedreturn portion of a light beam incident through an objective lens of anoptical pickup upon any one of a plurality of recording layers formedone over another in an optical disc, wherein the plurality of recordinglayers have different respective reflectances; and identifying, based ona peak level of the sum signal, which one of the plurality of recordinglayers formed in the optical disc has focused thereon the light beamfrom the optical pickup by comparing the peak level detected by thephotodetector to pre-set upper and lower thresholds set for each of theplurality of recording layers.
 23. The optical disc recording and/orplayback method as set forth in claim 22, wherein the identification iseffected after moving the objective lens optical-axially thereof so thatthe light beam is focused near any one of the plurality of recordinglayers formed in the optical disc.
 24. The optical disc recording and/orplayback method as set forth in claim 23, wherein it is judged whetherthe sum signal from the signal generator is below a predeterminedthreshold for any one of the plurality of recording layers in theoptical disc.
 25. The optical disc recording and/or playback method asset forth in claim 24, wherein it is identified which one of theplurality of recording layers in the optical disc has focused thereonthe light beam from the optical pickup, depending on whether when afocus error signal from the signal generator is at a predeterminedsignal level, a level of the sum signal from the signal generator isbelow a threshold for any one of the plurality of recording layersformed in the optical disc.
 26. The optical disc recording and/orplayback method as set forth in claim 25, wherein it is identified whichone of the plurality of recording layers in the optical disc has focusedthereon the light beam from the optical pickup, depending on whetherwhen the focus error signal from the signal generator crosses a signalzero level, the level of the sum signal from the signal generator isbelow a threshold for any one of the plurality of recording layersformed in the optical disc.